The 1970 BOCA code section 414.11 requires a door panel that with a 3/4 hour fire resistance rating or a 1 3/4" solid core wood door with no closer required. That might not be the applicable code but no other code would have been more restrictive and the 1972 CABO code would have been the same since this requirement is unchanged in later CABO codes.
All you should need to do is hire a carpenter to install a solid core wood door (20 min equivalent). If the code history is not convincing add the closer. I'm surprised a door from that era isn't solid core wood or metal clad.
Wish I knew how to edit comments.
I think you have to be a Sawhorse,, Forum supporting person,,, to edit.
Which we welcome, plus the cost ,,, will save you money on a project down the road, When you have one of those I don't know questions